The present invention relates to a system such as disclosed in the commonly owned application identified above, or more generally to systems having those operating characteristics of such system which will be discussed here. In particular, in systems of the type in question, first and second photosensitive means receive reflected radiation which has been emitted towards a subject and yield first and second photodetector signals which are integrated to yield first and second integral signals. The rate of change of one or the other of the two integral signals is greater, depending upon the polarity or direction of the state-of-focus error. In the system disclosed in the above-identified application, when one or the other of the two integral signals reaches a certain value, this initiates counting by a tolerance-range counter whose counting cycle duration allows an interval of extra time during which the other integral signal can attempt to reach a predetermined value too. If the other signal reaches such value within the extra interval of time, then the state-of-focus error, although non-zero, is of acceptably small magnitude, and output flip-flops, one per photodetector signal, assume states the same as if both integral signals reached such values simultaneously. If within the allotted tolerance-range interval the more slowly changing integral signal does not succeed in reaching its associated threshold level, then the output flip-flops assume states indicating which one of the integral signals has in fact reached such value, this then indicating the polarity of the state-of-focus error.